The influence of inner material with different average thermal conductivity on the performance of whole insulation system for liquid hydrogen on orbit storage

At present, several composite insulation systems were proposed that can be used for passive insulation systems, including foam-variable density multilayer insulation (VDMLI), aerogel-VDMLI and hollow glass microspheres (HGMs)-VDMLI. The passive insulation systems with different inner material (IM) showed different performances. However, the relationship between the average thermal conductivity of IM and the insulation performance of the whole system has rarely been investigated. It is of great significance for efficient configuration and matching of the passive insulation system. In this paper, a series of average thermal conductivity of IM were assumed to predict the insulation performance of the whole system at 20 K–300 K and high vacuum. In order to further illustrate the relationship between IM and MLI/VDMLI, the foam was replaced by the HGMs as 5 mm a unit forming a series of HGMs-foam-MLI/VDMLI insulation systems. The performance of the systems was investigated. After the foam was completely replaced by the HGMs, the performance of MLI and VDMLI systems was improved 33% and 13%, respectively. Moreover, each mode of heat transfer including solid conduction, radiation, and gas conduction for foam-MLI/VDMLI and HGMs-MLI/VDMLI insulation systems were calculated and analyzed.     

A novel cryogenic insulation system of hollow glass microspheres and self-evaporation vapor-cooled shield for liquid hydrogen storage

Liquid hydrogen (LH₂) attracts widespread attention because of its highest energy storage density. However, evaporation loss is a serious problem in LH₂ storage due to the low boiling point (20 K). Efficient insulation technology is an important issue in the study of LH₂ storage. Hollow glass microspheres (HGMs) is a potential promising thermal insulation material because of its low apparent thermal conductivity, fast installation (Compared with multi-layer insulation, it can be injected in a short time.), and easy maintenance. A novel cryogenic insulation system consisting of HGMs and a self-evaporating vapor-cooled shield (VCS) is proposed for storage of LH₂. A thermodynamic model has been established to analyze the coupled heat transfer characteristics of HGMs and VCS in the composite insulation system. The results show that the combination of HGMs and VCS can effectively reduce heat flux into the LH₂ tank. With the increase of VCS number from 1 to 3, the minimum heat flux through HGMs decreases by 57.36%, 65.29%, and 68.21%, respectively. Another significant advantage of HGMs is that their thermal insulation properties are not sensitive to ambient vacuum change. When ambient vacuum rises from 10⁻³ Pa to 1 Pa, the heat flux into the LH₂ tank increases by approximately 20%. When the vacuum rises from 10⁻³ Pa to 100 Pa, the combination of VCS and HGMs reduces the heat flux into the tank by 58.08%–69.84% compared with pure HGMs.     

Performance analysis of vapor-cooled shield insulation integrated with para-ortho hydrogen conversion for liquid hydrogen tanks

A composite thermal insulation system consisting of variable-density multi-layer insulation (VDMLI) and vapor-cooled shields (VCS) integrated with para-ortho hydrogen (P-O) conversion is proposed for long-term storage of liquid hydrogen. High-performance thermal insulation is realized by minimizing the thermal losses via the VDMLI design and fully recovering the cold energy released from the sensible heat and P-O conversion of the vented gas. Effects of different design considerations on the thermal insulation performance are studied. The results show that the maximum reduction of the heat leak with multiple VCSs can reach 79.9% compared to that without VCS. The heat leak with one VCS is reduced by 61.1%, and further reduced by 11.6% after adding catalysts. It is found that the deterioration of the insulation performance has an almost linear relationship with catalytic efficiency. A unified criterion with relative optimization efficiency is finally proposed to evaluate the improvement of the VCS number.     

Feasibility analysis of a new thermal insulation concept of cryogenic fuel tanks for hydrogen fuel cell powered commercial aircraft

Of cryogenic liquid hydrogen tanks for future airliners, their volumetric and gravimetric efficiencies, their robustness and their environmental adaptability are all strengthened via a novel thermal insulation concept proposed in this work.A conventional cryogenic tank is insulated either purely by a layer/layers of Polyurethane (PU) foam or by a vacuum-based multilayer insulation (MLI). In the new concept, an extra layer is inserted into the PU foam. The intermediate layer can be filled with liquid nitrogen while on the ground or with ambient air during flight.By this new design, analysis shows an approximate 33% volumetric saving compared to PU insulation. Furthermore, a 6-fold amount of passive heat input during cruise flight is easily achieved compared to the rest two concepts. This showcases an increased robustness against possible failure of the tank's active heating system, and the potential for significant parasitic power loss reduction.     

含二氧化硅气凝胶和相变材料三层玻璃窗热性能分析

为了研究含二氧化硅气凝胶和相变材料三层玻璃窗对严寒地区建筑能耗的影响,建立了相变材料层与其他透明壁层结合发生的传热数值模型。分析了含二氧化硅气凝胶和相变材料三层玻璃窗在不同二氧化硅气凝胶厚度、导热系数和不同保温材料下的动态热调节性能,得到了含二氧化硅气凝胶和相变材料三层玻璃窗内表面热流密度和液相率随时间的变化规律。结果表明:随着二氧化硅气凝胶厚度增加,总传热量降低和液相率增加,当二氧化硅气凝胶厚度为20～30 mm时,可以实现有效的利用太阳能;随着二氧化硅气凝胶导热系数增加,总传热量升高和液相率降低;当二氧化硅气凝胶的导热系数从0.022降低到0.014 W/(m·K)时,最大液相率从0.83增加到1.00。二氧化硅作为保温层比相变材料作为保温层具有更好的保温隔热作用。     

真空隔热油管传热性能研究

真空隔热油管是稠油注蒸汽开采的主要设施之一,其隔热性能直接影响热采效果,因此,分析隔热油管隔热层内部的传热过程,研究隔热层各种结构参数对隔热油管隔热性能的影响,对改善隔热油管隔热性能有重要的指导作用。本文在测试隔热油管视导热系数的实验模型基础上,建立了隔热层传热的物理及数学模型,计算得到不同结构参数下隔热油管的视导热系数;研究了玻璃丝布孔隙比、隔热层层数、支撑材料导热系数以及铝箔发射率对隔热油管隔热性能的影响,研究表明:隔热层层数宜选为4~6层之间;选用的铝箔发射率应在0.01~0.05之间;在缠绕玻璃丝布工艺中,尽量不要让玻璃丝布对角线方向受力,以确保较大的孔隙比;在隔热油管加工过程中,应尽量使玻璃丝布和铝箔保持干燥。     

Measurements of heat transfer of multi-layered wall construction with foam gypsum

A new wall structure and its manufacturing technique have been developed. With this technique an experimental wall fragment has been manufactured. It consists of the following layers: internal finishing layer (gypsum boards and vapour insulation), foam gypsum, thermal insulation (polystyrene) and a plaster outer layer. The heat transfer coefficient through the given wall has been determined. The construction element’s heat transfer coefficient (U) was determined by applying specially developed equipment and software. According to the experimental test, the coefficient’s U value for the multi-layer construction with the foam polystyrene thermal insulation is 0.36 ± 0.10 W/m² K.     

Field measurements on a district heating pipe with vacuum insulation panels

In Swedish district heating networks, around 10% of the supplied thermal energy is lost in the distribution system. One solution to decrease the losses is to use hybrid insulation district heating pipes, a concept where the innermost part of the thermal insulation consists of vacuum insulation panels, held in place by polyurethane foam. One problem with vacuum insulation panels are their sensitivity to high temperatures. This paper presents field measurements on a hybrid insulation district heating pipe where the temperatures have been measured continuously at various positions of a pipe section. The measurements show consistency and a large difference between hybrid insulation parts and reference parts without vacuum insulation panels. A superposition model has been used to calculate the temperature in a point and compare it to the measurement. The results are compared to the same calculation on the results from finite element simulations. The results show clearly that the vacuum panels in the pipes have not collapsed. A slow deterioration of the panels is harder to find with this model. Changes in the system, such as a return temperature which decreases over time, can give a larger impact, concealing the change in the panel performance.     

Application demonstration and performance of a cellulose triacetate polymer film based transparent insulation wall heating system

For the application demonstration of cellulose triacetate (CTA) polymer film based transparent insulation (TI) structures a technically and ecologically optimized TI facade system was developed and used to equip a south-oriented wall of a solar house meeting passive house standard in Graz, Austria. The demonstration building was equipped with an appropriate data recording system for solar irradiation, temperature, heat flux and humidity. The practical experiences within the heating periods 2002/03 and 2003/04 are reported in this paper. For the optimized TI facade system a solar energy efficiency of about 43% and a U-value of 0.76 W/(m² K) were obtained. Although CTA absorbs a high amount of water no adverse condensation phenomena were observable visually. The reasoning for these findings is explained and related to construction details.     

Single-phase heat transfer in the high temperature multiple porous insulation

An experimental study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of ‘Magnox’ nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. A large scale experimental test facility, based on the guarded hot plate method, was used for measuring the effective thermal conductivity of Magnox reactor pressure vessel insulation, which consists of alternate layers of plain steel foil and dimpled foil. The measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The experimental conditions corresponded to a heat flux of 75–1000 W/m², fluid pressures of atmospheric to 5 bar gauge, pack orientations in range of 0°–45° relative to the horizontal, leakage velocities ranging from 0.05 m/s to 0.20 m/s and inlet air bulk temperatures ranging from 18 °C to 290 °C. Local values of effective thermal conductivity of 0.04–0.23 W/m K were obtained for the above experimental conditions. The heat transfer modes in the insulation pack were conduction through the contacting metallic foils, thermal radiation across the gas gaps, and conduction and convection in the air. The effective thermal conductivity of the porous insulation increased with increasing air pressure, inclination angle, and air velocity. Buoyancy effects increased with increasing inclination angle and air pressure.     

Vacuum insulation properties of phenolic foam

Characteristic properties of phenolic foam as the interstitial material of a vacuum insulation panel are investigated experimentally. For the measurement of effective thermal conductivity, a vacuum guarded hot plate (VGHP) apparatus is used and the conductivity is measured at various vacuum levels. Radiative properties are found using a Fourier transform infrared spectroscopy (FT-IR) device. Solid conductivity is estimated using the porosity of the foam. Effective thermal conductivity at high level of vacuum is measured to be 5 mW/m K which is sum of solid conductivity (2.56 mW/m K) and radiative conductivity (2.44 mW/m K) with 5% of measurement uncertainty. The pore size of the foam is estimated to be 260 μm using rarefied gas conduction theory. This ensures insulation performance of phenolic foam up to about 10^?3 atm. Other practical characteristics of phenolic foam as the VIP core material are also discussed.     

Development of a multiple layer vacuum insulation chip

Advent of micro thermal devices such as lab-on-a-chip and micro heat pump necessitates development of highly effective insulation chips or layers. This paper reports the development of a vacuum insulation chip (VIC) having very low effective thermal conductivity and very small thickness. Fifty nanometer thickness metal coating on both sides of an LCD glass chip and 5 μm vacuum gap are stacked in a series to decrease the heat transfer by radiation. An array of support legs is necessary to maintain the structure under the atmospheric pressure. Design of VIC involves trade-offs between the heat conduction through the multi-layer structure and the mechanical strength. A model to determine the actual design values is proposed. The results are in reasonable agreement with the more refined results using commercial numerical codes. Based on these results, a VIC of 32 × 32 × 1.88 mm³ is manufactured, and the effective thermal conductivity is measured by guarded hot plate method. The chip shows effective thermal conductivities of 0.0015 and 0.001 W/m K at vacuum levels of 1.33 and 0.24 Pa (N/m²), respectively.     

The effects of PE additive on the performance of polystyrene vacuum insulation panels

The effects of adding polyethylene (PE) in polystyrene (PS) foaming material on the cell structure and the heat transfer of vacuum insulation panels (VIPs) are examined in this study. Several parameters are proposed to describe the foam structure, namely, the broken cell ratio, the average cell size and the solid volume fraction. Adding 2% PE was effective in altering the cell structure and reducing the heat transfer, while adding 5% PE did not improve the performance further. The lowest thermal conductivity found in this study is 4.4 mW m^?1 K^?1, which is among the best published performances of VIP.     

Numerical and experimental investigation on the thermal insulation performance of low temperature cold box

A new modeling method with the discrete ordinate (DO) model and GKT model was proposed to simulate the thermal insulation performance of a low temperature cold box. Experimental data from the thermal insulation experiment of a cold box were used to validate the model. The thermal insulation performance and the coupled radiation and conduction heat transfer were analyzed at various pressures, shield numbers and shield positions. The results confirmed that the thermal insulation performance can be significantly improved by the addition of a thermal insulation shield. It was found that the effects of shield position on the thermal insulation performance are different at lower pressure (<3 × 10^− 2 Pa) from higher pressure. A practical application was presented by the usage of the new model in analyzing the performance of the cold box in a cold neutron source. A composite thermal insulation method was proposed to reduce the cold loss by 18.3% and 9% respectively compared with those with the aluminum foil wrapping and shield insulating methods.     

A vapor chamber using extended condenser concept for ultra-high heat flux and large heater area

We proposed an extended vapor chamber (EVC), consisting of an evaporator part and an extended condenser part. A layer of compressed copper foam was sintered on the inner evaporator surface. The extended condenser includes a circular-straight groove network and a fin region. The groove network distributes generated vapor everywhere in the internal volume of EVC. A set of capillary holes are machined within fins. A sliced copper foam bar is inserted in each of capillary hole. The peaks of copper foam bar are tightly contacted with the evaporator copper foam piece. Water is used as the working fluid with a heater area of 0.785 cm². A minimum thermal resistance of 0.03 K/W is reached for the bottom heating. The heat flux is up to 450 W/cm² without reaching dryout. The transition point of thermal resistances versus heat fluxes is significantly delayed with the heat flux exceeding 300 W/cm², beyond which thermal resistances are only slightly increased. EVC not only improves temperature uniformity on the evaporator and fin base surfaces, but also evens the temperature distribution along the fin height direction to increase the fin efficiency. Inclination angles and charge ratios are combined to affect the thermal performance of EVC. An optimal charge ratio of 0.3 was recommended. EVC can be used for ultra-high heat flux and larger heater area conditions.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

Economic and environmental impacts of insulation in district heating pipelines

The determination of optimum thickness of insulation is often applied to energy technologies and building projects. In this study, the energy, economic and environmental evaluations of thermal insulation in district heating pipeline are discussed. The optimum insulation thickness, energy saving over a lifetime of 10 years, payback period and emissions of CO₂, CO and SO₂ are calculated for nominal pipe sizes and fuel types based on heating loads in Afyonkarahisar/Turkey. The life cycle cost analysis is used to determine the optimum thickness of the pipeline material in order to take into account the change in inflation that directly affect both the cost of pipeline material and fuels depending on fuel type. The results show that the highest value of optimum insulation thickness, energy savings, emissions and the lowest payback period are reached for a nominal pipe size of 200 mm. About three times more energy saving results by making 200 mm nominal pipe instead of 50 mm. Considering the economical and environmental advantages, the geothermal energy is a better choice and then fuel-oil. When thermal insulation is done in a district heating pipeline, there will be a significant reduction of 21% in the amount of CO₂ emitted to the atmosphere.     

船舶管道多层隔热结构的传热机理分析和实验研究

采用能量平衡方程和P-1阶微分近似建立了船舶管道系统多层隔热结构的稳态传热模型.讨论了在船舶管道系统上采用多层隔热结构的可行性,并进行了相应的实验验证.结果表明,在硅酸铝纤维隔热材料层间添加金属铝箔纤维布能提高材料的隔热性能,且随着使用温度升高,提升幅度增大.对于中低温的管道系统,因隔热效果提升有限,不推荐采用多层隔热结构.